I have the oppertunity to buy one of these lenses used for about $5000 Australian. The lens nearly unused and in perfect condition. Is this a good price? I'm having trouble finding used copies of this lens online to compare the price with. 1) There doesn't really appear to be too many and 2) The search results get mixed with the version II lens. I really don't NEED the lens, though at this price it seems hard to pass up.